1. Field of the Invention
The present invention relates to a semiconductor device and a driving method thereof.
2. Description of the Related Art
A semiconductor device such as a processor includes a signal processing circuit having a variety of structures. A signal processing circuit generally includes a variety of memory devices such as a register and a cache memory. A register has a function of temporarily holding data for carrying out arithmetic processing, holding a program execution state, or the like. In addition, a cache memory is located between an arithmetic circuit and a low-speed main memory in order to reduce access to the main memory and speed up the arithmetic processing.
In a memory device such as a register or a cache memory, reading and writing of data needs to be performed at higher speed than in a main memory. Thus, in general, a flip-flop or the like is used as a register, and a static random access memory (SRAM) or the like is used as a cache memory. Such a register, a cache memory, and the like are volatile memory devices in which data is lost when supply of power supply potential is stopped.
In order to reduce power consumption, a method for temporarily stopping supply of power supply voltage to a signal processing circuit in a period during which data is not input or output has been suggested. For example, Patent Document 1 discloses a method in which data in a volatile memory device such as a register or a cache memory is stored in a nonvolatile memory device formed in the periphery of the volatile memory device so that data can be held also in a period during which supply of power supply potential is stopped.